Nuclear magnetic resonance as a tool for the assessment of postharvest changes in the metabolome of the skin of sweet cherry ( Prunus avium L.)

Abstract

A metabolomics approach based on nuclear magnetic resonance (NMR) spectroscopy, a rapid and simple technique, was used for the assessment of chemical changes of ‘Van’ sweet cherries during their storage. This approach affords a holistic analysis since NMR spectroscopy allows the simultaneous detection of diverse groups of secondary and primary metabolites, and reflects the real molar levels of the metabolites. Additionally, color parameters L*, a* and b*, total phenolic content (TPC), total flavonoid content (TFC), anthocyanidins and total antioxidant potential (AOP) were measured. After harvest, cherries were stored at 1°C and sampling was done after 0, 2, 4, 7 and 10 days. Cherry skin was removed, freeze dried and stored at -80°C till analyses. For NMR analyses, freeze dried cherry skins have been extracted with methanol-d4/trifluoroacetic acid-d solvent mixture. For other analyses, samples were extracted in 70% ethanol with 1% of formic acid. The 1D and 2D NMR spectral analysis confirmed that sugars (glucose, fructose, sucrose), organic acids (malic and citric acid), and phenolic compounds (neochlorogenic acid, cyanidin 3-O-rutinoside, and cyanidin 3-O-glucoside) are the main compounds in the cherry skin extracts. The 1H NMR spectral data have been subjected to multivariate analysis. Orthogonal projections to latent structures (OPLS) were applied to correlate the NMR data with the storage time. The changes in chemical composition among the samples with different storage time were clearly visible on the score plot. According to the goodness of fit of the model (R2=0.954), and its predictive ability (Q2=0.634), cherry skin extracts were well-correlated with the storage time. A positive correlation of the signals corresponding to glucose, neochlorogenic acid, cyanidin 3-O-rutinoside, and cyanidin 3-O-glucoside with the storage time of cherries has been observed on the loading plot. According to the loading plot, the increase in the content of glucose, neochlorogenic acid, cyanidin 3-O-rutinoside, and cyanidin 3-O-glucoside has been correlated with the storage time of cherries. Since polyphenolic compounds were found to correlate well with the storage time, the content of redox active compounds was further assessed with AOP, TPC and TFC. All these parameters were increasing for 7 days after harvest then started to decline slightly. The content of all three anthocyanidins increased 2 days after harvest then remained stable till the end of sampling period.